FIG. 1 (Prior Art) is a simplified top-down diagram of an image sensor integrated circuit 1. Image sensor integrated circuit 1 includes a valid area 2 surrounded by a shielded area 3. Image sensor 1 includes a two-dimensional matrix of sensor elements (the sensor elements are not illustrated). Each sensor element outputs an analog data value indicative of the intensity of light detected by the sensor element. The analog data values are read out of the image sensor, pixel by pixel, row by row. The analog data values are converted into corresponding digital data values by subsequent processing circuitry referred to here as an analog-front-end (AFE). If a sensor element is not illuminated, then the data value output from image sensor 1 for that sensor element should correspond to a digital zero in the AFE. This is often not the case in an actual image sensor. The AFE therefore reads out the values from shielded sensor elements to obtain an average value that sensor elements will output if they are not illuminated. The resulting “optical black level” or “OB level” within the AFE is then typically subtracted from the sensor element data output values from the valid pixel area 2 so that the corrected data values will have a digital zero value under dark conditions. As a row of sensor elements is read, the values from shielded sensor elements that are located to a side of the valid area may be accumulated in the AFE to obtain the OB level value.
FIG. 2 (Prior Art) is a diagram of a subsequent processing circuit that receives values output from an image sensor, amplifies the values by a gain, and subtracts an OB level from the amplified valid pixel area values. The clamp level calculation circuit determines the OB level from values read from shielded side window optical black detection areas along the sides of the image sensor. The OB level is subtracted from offset adjusted and amplified values read out from the valid area of the image sensor. See U.S. Pat. No. 6,304,292 for more specific information on the circuit of FIG. 2.
FIG. 3 (Prior Art) is diagram that illustrates an operation of a conventional digital still camera. The still camera is used in a low-resolution preview mode to compose a digital photograph to be captured. The preview mode involves the mixing of values read from multiple sensor elements. A first gain is therefore used in the preview mode. When the user presses a shutter button at time T1 in order to take the digital photograph, the processing circuit causes the image sensor to carry out a cleanup operation to remove unwanted charge that might be left in the readout circuitry of the image sensor. Because pixel mixing will not occur in the subsequent high-resolution readout, the gain is changed. The feedback loop that performs OB level adjustment then stabilizes between times T2 and T3 in what is sometimes referred to as a “black level transition period”. The time constant of the feedback loop is large because values from shielded sensor elements at the ends of many different sensor element rows may have to be processed in order to obtain an adequately accurate OB level.
When the feedback loop has stabilized (time T3), rows of values are read out from the image sensor as a sequence of three fields. As the rows of sensor values are read and processed, the OB level adjusting feedback loop operates and adjusts the OB level using the values read from the shielded areas to the sides of the valid area. There may be what are called “bad pixels” in these shielded side areas. A bad pixel typically outputs a constant value regardless of how the sensor element of the bad pixel is illuminated. If there are bad pixels in the optical black areas corresponding to one field and no such bad pixels in the optical black areas corresponding to other fields, then the OB level can be seen to jump significantly from field to field. This jumping in the OB level may be perceived as a flicker (sometimes appears as “line noise”) if the series of interleaved fields is viewed repeatedly. Even if there are no bad pixels in the side optical black areas, the OB level may vary during image sensor readout as the feedback loop operates. An improved and faster circuit is desired.